Mention inbound UDP 51820 in troubleshooting

Author: Silex

src/pages/how-to/troubleshooting-client.mdx

@@ -3,6 +3,12 @@
 This document offers practical tips and insights to help you debug various problems, ensuring a seamless user
 experience.
 
+## P2P / Relayed
+
+Netbird will use signal/stun to try to connect directly for a P2P connexion, if that is not possible it'll use the relay server.
+
+To help peers get a P2P connection, allowing inbound UDP to port 51820 to one of the peers helps a lot.
+
 ## NetBird agent status
 
 The netbird agent is a daemon service that runs in the background; it provides information about peers connected and